Let the git remote be set from Settings

Attaching a workspace to an existing repository meant leaving for a
terminal: gitsync could init, commit and push, and could tell whether an
'origin' existed, but had no way to create one, and nothing in the
settings held a remote URL. Sync just logged "no 'origin' remote
configured" and committed locally.

Settings > Git remote... now shows the workspace repository's origin and
lets it be set, repointed or cleared. The URL is read from and written
to the repository, never to config.json, because a remote belongs to the
checkout — so each workspace shows its own, and the box is refreshed
whenever the repository in play changes.

This is separate from the Gitea publishing in New project, which is a
cookiecutter post-gen hook and only ever applies to generated projects.

Also factors out first_output_line, which repo_root and the new
origin_url were both open-coding, and origin_args, so the add-vs-set-url
decision can be tested without a repository on disk.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBWj9TphFMCoh7VHaSRnvQ

src/app/ui.rs

@@ -424,6 +424,10 @@ impl App {
                         ui.close_menu();
                         self.show_project_settings = true;
                     }
+                    if ui.button("Git remote…").clicked() {
+                        ui.close_menu();
+                        self.open_git_remote();
+                    }
                     if ui.button("Manuscript details…").clicked() {
                         ui.close_menu();
                         self.show_manuscript_settings = true;